In recent years there has been increasing interest expressed in the development of microporous membranes of a semipermeable nature which are useful in separating the components of a solution. For instance, semipermeable membranes have been looked to as a possible means to demineralize or purify otherwise unusable water and to thereby alleviate the increasing demands for potable water necessitated by the rapid growth of the population and industry in many parts of the world. Separation techniques which employ such membranes include electrodialysis, reverse osmosis, ultrafiltration, ion exchange, etc.
Electrodialysis separations employ an electrolytic cell having alternating anionic and cationic membranes that collect desalted and concentrated solutions in adjacent compartments. Such a technique can be used to purify liquids by removing ionizable impurities, to concentrate solutions of electrolytes, or to separate electrolytes from nonelectrolytes.
As opposed to the charge dependent types of separations, reverse osmosis utilizes pressure to move materials, which may be either ionic or non-ionic, selectively through a membrane. Ultrafiltration, which is very similar, uses gravity or applied pressure to effect separation using membranes which act as submicronic sieves to retain large molecules and permit the passage of small, ionic or non-ionic forms.
In the prior art, semipermeable membranes have been formed from a variety of natural and synthetic materials including polybenzimidazoles. See, for instance, commonly assigned U.S. Pat. Nos. 3,699,038; 3,720,607; 3,737,042; 3,841,492; and 3,851,025. The polybenzimidazole membranes are recognized to possess superior thermal stability when compared with other semipermeable membranes, such as those formed from cellulose acetate.
A process for chemically modifying polybenzimidazole membranes in order to increase the toughness thereof is disclosed in commonly assigned U.S. Pat. No. 4,020,142. Imidazole groups of the polybenzimidazole membrane are crosslinked by reaction with a strong polybasic acid or its acid chloride. When an acid chloride is utilized, the polybenzimidazole tends to be covalently crosslinked through an amide-type linkage; otherwise, the crosslinking tends to be ionic in nature. The process does not involve sulfonation of the membrane.
U.S. Pat. No. 3,808,305 discloses a process for chemically crosslinking ionic membranes by covalent bonds. The crosslinking is accomplished during the formation of the membrane. Polybenzimidazoles are not specifically disclosed as suitable polymers for use in the process.
Technical reports published by Celanese Research Company (ASD-TR-73-49) and Fabric Research Laboratories (AFML-TR-73-29) disclose a process for sulfonating polybenzimidazole fibers in order to reduce the thermal shrinkage thereof.
It is an object of the present invention to provide a process for chemically modifying a preformed polybenzimidazole semipermeable membrane in order to produce a covalently bonded sulfonated polybenzimidazole semipermeable membrane.
It is also an object of the present invention to provide a process for chemically modifying a preformed polybenzimidazole semipermeable membrane in order to produce a covalently bonded sulfonated polybenzimidazole semipermeable membrane which exhibits increased water flux for reverse osmosis purposes.
It is also an object of the present invention to provide a process for chemically modifying a preformed polybenzimidazole semipermeable membrane in order to produce a covalently bonded sulfonated polybenzimidazole semipermeable membrane which exhibits increased resistance to fouling.
It is also an object of the present invention to provide a process for chemically modifying a preformed polybenzimidazole semipermeable membrane in order to produce a covalently bonded sulfonated polybenzimidazole semipermeable membrane which exhibits cation selectivity for electrodialysis and other electrochemical applications.
It is also an object of the present invention to provide a covalently bonded sulfonated polybenzimidazole semipermeable membrane.
These and other objects, as well as the scope, nature, and utilization of the present invention, will be apparent from the following description and the appended claims.